Blood, Vol. 96 No. 2 (July 15), 2000:
pp. 532-539
Surface expression of glycoprotein Ib
is dependent on
glycoprotein Ib
: evidence from a novel mutation causing
Bernard-Soulier syndrome
Niamh Moran,
Patricia A. Morateck,
Adele Deering,
Michelle Ryan,
Robert R. Montgomery,
Desmond J. Fitzgerald, and
Dermot Kenny
From the Department of Clinical Pharmacology, Royal College of
Surgeons in Ireland, Dublin, Ireland, and the Blood Research Institute,
the Blood Center of Southeastern Wisconsin, Milwaukee, WI.
Bernard-Soulier syndrome is a rare bleeding disorder caused by a
quantitative or qualitative defect in the platelet glycoprotein (GP)
Ib-IX-V complex. The complex, which serves as a platelet receptor for
von Willebrand factor, is composed of 4 subunits: GPIb
, GPIb
,
GPIX, and GPV. We here describe the molecular basis of a novel form of
Bernard-Soulier syndrome in a patient in whom the components of the
GPIb-IX-V complex were undetectable on the platelet surface. Although
confocal imaging confirmed that GPIb was not present on the platelet
surface, GPIb was readily detectable in the patient's platelets.
Moreover, immunoprecipitation of plasma with specific monoclonal
antibodies identified circulating, soluble GPIb. DNA-sequence
analysis revealed normal sequences for GPIb and GPIX. There was a G
to A substitution at position 159 of the gene encoding GPIb,
resulting in a premature termination of translation at amino acid 21. Studies of transient coexpression of this mutant, W21stop-GPIb,
together with wild-type GPIb and GPIX, demonstrated a failure of
GPIX expression on the surface of HEK 293T cells. Similar results were
obtained with Chinese hamster ovary IX cells, a stable cell line
expressing GPIb that retains the capacity to re-express GPIX. Thus,
we found that GPIb affects the surface expression of the GPIb-IX
complex by failing to support the insertion of GPIb and GPIX into
the platelet membrane.
